Method and apparatus for differentiating battery types

Abstract: The cell type of a battery (82) is determined by characterizing the series resistance behavior of various cell types to determine ranges (40, 48) of series resistance values exhibited by batteries of a particular cell type. The series resistance of a battery (82) of unknown cell type is then measured. If the series resistance measurement is determined to be within a range (40, 48) of series resistance values exhibited exclusively by a cell type, the battery is identified as having that cell type. However, for batteries whose series resistance measurements are within a non-exclusive range (52) of series resistance values, a second cell type discharge characteristic such as battery voltage is also characterized to differentiate between the cell types. The second discharge characteristic of these batteries is also measured and used to identify the cell type.

Description: FIELD OF THE INVENTION The present invention relates to a circuit and method for identifying the cell type of a battery, and more particularly relates to a circuit and method for identifying a battery's cell type while in circuit and operating. BACKGROUND AND SUMMARY OF THE INVENTION Batteries using various cell technologies (cell types) have been developed and are in widespread use. Two of the most common cell types are alkaline and nickel-cadmium (Ni-Cd). Other cell types include nickel-metal-hydride and various lithium technologies. In many applications, batteries of differing cell types are interchangeable. For example, in many household electronic products such as calculators, palmtop computers and the like, both alkaline and Ni-Cd type batteries can be used. In some circumstances, however, it is necessary to differentiate between the cell types of the batteries being used in a particular application. For example, in some palmtop computers, an end-of-life or state-of-charge procedure is used to gauge the remaining energy in the battery and provide an indication of same to the user. Typically, these procedures are dependent on the voltage characteristic of the battery over its discharge cycle. Because the voltage characteristic of batteries differs between cell types, the cell type of the battery must be known to accurately gauge the remaining battery energy. If a battery is compared to a voltage characteristic of another cell type, the measurement of the remaining energy will be in error. Improper cell type usage could result in the diagnosis of a fully charged battery as having insufficient remaining charge. The user might prematurely discard or unnecessarily recharge such a misdiagnosed battery. Improper cell type usage could also result in failing to diagnose the impending end of life of a battery with little remaining charge. The user, relying on such diagnosis, could lose valuable data on the eventual failure of the battery. Cell type differentiation is also critical to battery recharging. Some cell types, such as Ni-Cd, are rechargeable, while others, such as alkaline, are not. If a recharging current is applied to a non-rechargeable battery, the temperature of the battery increases rapidly as does the internal pressure. Eventually, if recharging is continued, the battery will vent, resulting in leakage of caustic chemicals from the battery and possibly an explosion. Currently, many battery operated products simply rely on the user as directed by an owner's manual or warnings attached to the battery operated product to select and employ batteries of the proper cell type. Some palmtop computers may even employ user interrogation to identify the cell type prior to gauging the battery's state-of-charge. However, considering the possible consequences of improper cell type usage, it is desirable to have a system to protect the unwary user of a battery operated product from using batteries of the wrong cell type. Manufacturers of battery operated products have also relied on mechanical lock-out schemes to protect against improper cell type usage. Typical mechanical lock-out devices protect against improper cell type usage by preventing installation of batteries that do not have a particular size, shape or other mechanical feature. Only batteries having the particular mechanical feature can be installed. Typically, the mechanical feature is of non-standard design to ensure that only batteries of the desired cell type have that mechanical feature. However, the use of non-standard batteries usually results in limited availability of suitable batteries to the public. Products employing these schemes may therefore have difficulty in gaining public acceptance. Further, lock-out schemes are not useful to palmtop computer state-of-charge gauges, where the use of interchangeable cell types is desired but the particular cell type in use must be known. The ability to identify the cell type of a battery while the battery is in use is therefore needed. The present invention provides an apparatus and method for identifying the cell type of a battery preferably while it is in circuit and operating. The invention uses the series resistance of the battery to identify its cell type. Batteries of different cell types have different discharge characteristics for a particular method of discharge. Most electronics systems use a pulsed, constant power discharge method (i.e., a constant power is intermittently drawn from the battery). Other discharge methods include constant load impedance, constant current and constant power discharge. It is possible to characterize the behaviour of various cell types under a specific discharge method and specific end use. The characteristic behaviour of a cell type in a specific use can then be applied to differentiate between cell types. Published International Patent Application No. WO 92/17791 describes a system for sorting used batteries by measuring various ferromagnetic characteristics thereof. EP-A-0432689 describes a system for determining both the type of a battery and the power remaining. In this system, the open-circuit voltage of the battery is determined by measuring the voltage drop across two different known resistors which are placed alternately in circuit with the battery. One particular cell discharge characteristic, the series resistance, is distinct for various cell types under pulsed, constant discharge usage. For example, the series resistance of alkaline batteries falls within a certain range through its life cycle. The series resistance of Ni-Cd batteries, however, falls within a second range of values that, for the most part, is distinct from that of alkaline batteries. The present invention uses this series resistance behavior of batteries to distinguish the cell type. In the case of some cell types, however, the series resistances of the cell types are not distinct over their full discharge cycle. For example, alkaline and Ni-Cd cell types both exhibit series resistances in a certain intermediate range of values in different portions of their life cycles. In such cases, it is also necessary to use a second discharge characteristic, such as battery voltage, to differentiate between cell types. When the cell types exhibit series resistance values in the intermediate range, the battery voltage or other discharge characteristic of the cell types fall into differing, distinct ranges. In accordance with a first aspect of the present invention, there is provided a method of determining a battery's type, as defined in claim 1. In accordance with a second aspect of the present invention, there is provided an apparatus for differentiating battery cell types, as defined in claim 8. In one embodiment of the present invention, the battery test circuit measures the series resistance of a battery by pulsing the battery with a current pulse of known magnitude and measuring the change in the battery voltage produced by the pulse. The series resistance is the ratio of the voltage change to the magnitude of the current pulse. The testing circuit measures the battery voltage by sampling the voltage and digitizing the sampled voltage. The cell type identification circuit is preferably embodied in a microprocessor programmed to execute the method of the present invention, but may also be embodied in dedicated hardware. The identification circuit receives the series resistance and voltage measurements from the testing circuit and compares the measurements to established ranges of battery behavior for the cell types. The identification circuit then identifies the battery as being of the cell type associated with the range in which the measurements lie. Additional features and advantages of the invention will be made apparent from the following detailed description of a preferred embodiment, which proceeds with reference to the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a graph of the series resistance of alkaline and Ni-Cd batteries over their charge state profile (percentage remaining charge in the battery). Fig. 2 is a graph of the battery voltage as a function of the percentage remaining charge of alkaline and Ni-Cd batteries whose series resistance is within an intermediate range in the graph of Fig. 1. Fig. 3 is a block diagram of a circuit for determining and identifying battery cell type according to a preferred embodiment of the present invention. Fig. 4 is a block diagram of a battery testing circuit in the circuit of Fig. 3. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The preferred embodiment of the present invention distinguishes between two common interchangeable battery cell types, alkaline and Ni-Cd, using a measurement of a battery's series resistance. Before the series resistance measurement can be used to identify the cell type of the battery, it is first necessary to characterize the series resistance behavior of the cell types for a particular discharge method. The series resistance characteristic of the cell types can be determined empirically by observation of a cell type's behavior during discharge or from a battery manufacturer's specifications. When subjected to the pulsed, constant power discharge typical of electronic circuits, alkaline and Ni-Cd batteries each have distinct series resistance characteristics. With reference to Fig. 1, the average series resistance of alkaline type batteries as a function of their state-of-charge (measured as a percentage of full charge remaining) graphs as a downward sloping curve 20. The maximum and minimum series resistance values exhibited by alkaline batteries as a function of state-of-charge are shown by downward sloping curves 22, 24 respectively on each side of the average alkaline series resistance curve 20. At any given state-of-charge between empty (0% charge remaining) and fully charged (100% of charge remaining), the series resistance of alkaline batteries falls between the maximum curve 22 and the minimum curve 24. The average series resistance of Ni-Cd batteries as a function of their state-of-charge is graphed by a second downward sloping curve 28 below the minimum alkaline series resistance curve 24. Ni-Cd batteries exhibit a characteristic series resistance that falls between a maximum Ni-Cd series resistance curve 30 above the average Ni-Cd series resistance curve 28 and a minimum Ni-Cd series resistance curve 32 below the average curve 28. A next step in the determination of a battery's cell type is to identify ranges of values exhibited exclusively by particular cell types. In the preferred embodiment of the invention, exclusive series resistance ranges are identified for both alkaline and Ni-Cd cell types. In the graph of Fig. 1, a maximum series resistance value 36 exhibited by Ni-Cd batteries is at a point 38 along the maximum Ni-Cd series resistance curve 30 at 0% remaining charge. The series resistance values above the maximum Ni-Cd series resistance value 36 are exhibited exclusively by alkaline batteries. Thus, in the preferred embodiment of the invention, the maximum Ni-Cd series resistance value 36 forms a lower limit of a range 40 of series resistance values exhibited exclusively by alkaline batteries. A minimum series resistance value 44 exhibited by alkaline batteries lies on the minimum alkaline series resistance curve 24 at 100% charge remaining (point 46). Series resistance values below the minimum alkaline series resistance value 44 are exhibited exclusively by Ni-Cd batteries. The minimum alkaline series resistance value 44 thus forms an upper limit of a range 48 of series resistance values exhibited exclusively by Ni-Cd batteries. The minimum alkaline series resistance 44 (Ni-Cd range upper limit) is less than the maximum Ni-Cd series resistance 36 (alkaline range lower limit). Between the limits 36, 44 of the alkaline and Ni-Cd ranges is an intermediate range 52 of series resistance values exhibited by both alkaline and Ni-Cd cell types. Alkaline batteries, however, exhibit series resistance values within the intermediate range when at or near full charge, whereas Ni-Cd batteries exhibit series resistance values in the intermediate range when discharged or nearly discharged. The cell type of a battery is determined by measuring the series resistance of the battery and comparing the measured series resistance to the limits 36, 44 of the exclusive ranges 40, 48. The battery's series resistance is first compared to the alkaline range lower limit 36. Series resistances greater than the lower limit 36 lie in the alkaline range 40. Therefore, if the battery's series resistance is greater than the lower limit 36, the battery must be alkaline. However, if the battery's measured series resistance is less than the alkaline range lower limit 36, the battery's series resistance is also compared to the Ni-Cd range upper limit 44. Batteries with series resistances less than the Ni-Cd range upper limit 44 (i.e., the series resistance is within the Ni-Cd range 48) are Ni-Cd type batteries. When the series resistance of the battery lies in the intermediate range 52 (less than the alkaline range lower limit 36 but greater than the Ni-Cd range upper limit 44), the battery could be either alkaline or Ni-Cd. To determine the cell type of a battery having a series resistance within the intermediate range, a further discharge characteristic of alkaline and Ni-Cd batteries must be considered. In the preferred embodiment of the invention, the voltage characteristic of alkaline and Ni-Cd cell types is used to differentiate between batteries having a series resistance in the intermediate range 52. By characterizing the voltage of alkaline and Ni-Cd batteries whose series resistance falls within the intermediate range, a limit can be determined to differentiate between the cell types. With reference to a graph of the battery voltage versus state-of-charge shown in Fig. 2, alkaline batteries having series resistances within the intermediate range 52 (Fig. 1) have a remaining charge greater than a minimum (alkaline) state-of-charge 64 near 100% remaining charge. Ni-Cd batteries, however, with a series resistance in the intermediate range 52 (Fig. 1) have a remaining charge less than a maximum (Ni-Cd) state-of-charge 66 near complete discharge (0% remaining charge). Alkaline batteries whose remaining charge is above the minimum alkaline charge 64 exhibit battery voltages above a minimum alkaline voltage 68. Ni-Cd batteries whose remaining charge is less than the maximum Ni-Cd state-of-charge 66 exhibit battery voltages less than a maximum Ni-Cd voltage 70. The maximum Ni-Cd voltage 70 is below the minimum alkaline voltage 68. Accordingly, in the preferred embodiment of the invention, a battery voltage limit 72 between the maximum Ni-Cd voltage 70 and the minimum alkaline voltage 68 is selected to differentiate between those battery voltages exhibited by alkaline batteries with series resistances in the intermediate range 52 (alkaline voltage range 74) and those battery voltages exhibited by Ni-Cd batteries with series resistances in the intermediate range (Ni-Cd voltage range 76). The cell type of batteries whose measured series resistance is within the intermediate range 52 is determined using the battery voltage limit 72. When a battery is determined to have a series resistance in the intermediate range, the battery's voltage is also measured. The measured battery voltage is compared to the battery voltage limit 72. Batteries whose measured battery voltage is greater than the battery voltage limit 72 are alkaline type batteries. If, however, the measured battery voltage is less than the battery voltage limit 72, the battery is a Ni-Cd type battery. Once the cell type of a battery is determined according to this method, the cell type is identified to the user or to another circuit needing the information. For example, the cell type of the battery can be identified to the user by illuminating an LED corresponding to the determined cell type. The cell type can also be identified to another circuit such as a shut-off circuit in a battery recharger by providing a signal corresponding to the determined cell type of the battery being recharged. The shut-off circuit could then prevent recharging of a battery determined to have a non-rechargeable cell type. Referring to Fig. 3, a circuit 80 for determining the cell type of a battery 82 according to the preferred embodiment of the invention comprises a battery testing circuit 86 and a cell type identification circuit 88. In the illustrated embodiment, the battery 82 is installed in a battery operated product to supply power to a load 90. The circuit 80 is incorporated in the battery operated product to provide an identification of the battery type with an id signal at output 92. The battery testing circuit 86 is connected to the battery 82 and measures the series resistance and battery voltage of the battery 82. With reference to Fig. 4, the battery testing circuit 86 preferably comprises a constant current source 96 coupled to the battery 82 through a current switch 102. The current switch 102 closes responsive to a current pulse enable signal applied at an input 104 to connect the current source 96 to the battery 82. By applying the current pulse enable signal at the input 104, the battery 82 can be selectively driven with a current of known magnitude supplied by the current source 96. The battery testing circuit 86 also comprises a conventional voltage sample and hold circuit 108 which is AC coupled to the battery through a capacitor 110. Responsive to a sample enable signal applied at an input 112, the voltage sample and hold circuit 108 samples a voltage at a voltage input 114 and provides the sampled voltage to an output 116. The output 116 is coupled through a switch 120 to an analog to digital converter (ADC) 122. The switch 120 selectively connects an analog voltage input 124 of the ADC 122 to the output 116 of the voltage sample and hold circuit 108 or to the battery 82 responsive to a test select signal applied to an input 126. The ADC 122 provides a digital value at an output 130 corresponding to an analog signal at its input 124. When measuring the series resistance of the battery 82, the switch 120 is operated to connect the output 116 of the voltage sample and hold circuit 108 to the ADC 122 by applying the test select signal for a series resistance measurement at the input 126. The current pulse enable signal is applied to the input 104 for a short time so that the battery 82 is driven with a current pulse of known magnitude. After the current pulse has stabilized at the known magnitude, the sample enable signal is applied at the input 112. Since the input 114 of the voltage sample and hold circuit 108 is AC coupled to the battery 82, the voltage at the input 114 when the current pulse stabilizes is a difference in the voltage of the battery 82 caused by driving the battery with the current pulse. Thus, when the sample enable signal is applied, the voltage sample and hold circuit 108 samples the battery voltage difference caused by the current pulse. This battery voltage difference is related to the series resistance by the following equation: R = Δ V Δ I where R is the series resistance, delta V is the battery voltage difference, and delta I is the current pulse magnitude. The battery voltage difference is converted to a digital value by the ADC 122 and provided as a scaled measurement of the series resistance at the output 130. When measuring the voltage of the battery 82, the test select signal is applied at the input 126 to connect the battery 82 to the input 124 of the ADC 122 with the switch 120. The ADC 122 then forms a digital value at the output 130 proportional to the battery voltage. The cell type identification circuit 88 determines the cell type of the battery 82 from the series resistance and battery voltage measurements according to the method of the present invention. The cell type identification circuit 88 can comprise one or more comparators for comparing the series resistance measurement to the predetermined limits 36, 44 of the alkaline and Ni-Cd ranges 34, 48 and the battery voltage measurement to the battery voltage limit 72. The cell type identification circuit 88 would also comprise a logic circuit for determining the cell type from the comparisons of the measurements to the limits and for forming a digital value corresponding to the cell type at the output 92. In a preferred embodiment of the invention, however, the circuit 80 is included in a palmtop computer to determine the cell type of the battery 82 prior to gauging the battery's state-of-charge. In such an embodiment, the cell type identification circuit 88 is preferably implemented in software and carried out by the computer's microprocessor. Under control of the software, the microprocessor generates the test select, current pulse enable, and sample enable signals to produce the series resistance and battery voltage measurements with the battery testing circuit 86. The microprocessor then reads the measurements from the battery testing circuit and compares the measurements to the predetermined limits 36, 44, and 72. In accordance with the method of the present invention, the microprocessor then determines a digital value corresponding to the cell type of the battery 82. The cell type value can then be used by the microprocessor in a subsequent state-of-charge gauge procedure. Having described and illustrated the principles of my invention with reference to a preferred embodiment, it will be recognized that the invention can be modified in arrangement and detail without departing from such principles. For example, in the illustrated embodiment of the invention, a battery's series resistance and voltage measurements are compared to various limits to determine if the measurement is within a range of values exhibited by a particular cell type. In other embodiments of the invention, other means for determining whether a measurement is within a particular range can be used. One such means is a look-up table which associates a plurality of possible measurement values with particular ranges. After measuring the series resistance or battery voltage, the range in which the measurement lies is simply read from the table. The look-up table could be further modified to associate a pair of series resistance and voltage measurements with the cell type of a battery having those measurements. In a further embodiment, the determination of whether a battery's series resistance and voltage measurements are associated with a particular cell type is made by calculating a function (e.g. the product) of the series resistance and battery voltage measurements and comparing the result to exclusive ranges of the function exhibited by various cell types.

Claims: A method of determining a battery's type from a first and a second candidate cell type which exhibit a common range of series resistance values, the battery having positive and negative output voltage terminals, the method comprising: measuring the series resistance of the battery between the positive and negative output voltage terminals thereof; determining whether the series resistance of the battery lies within a first predetermined range of series resistance values exhibited by batteries of both the first and second cell types; when the series resistance of the battery is determined to lie within the first predetermined range of series resistance values exhibited by batteries of both the first and second cell types, the method further comprising the steps of: measuring the voltage of the battery between the positive and negative output voltage terminals thereof; determining whether the voltage of the battery lies within a second predetermined range of voltages exhibited by batteries of the first cell type whose series resistance is within the first predetermined range of series resistance values; determining whether the voltage of the battery lies within a third predetermined range of voltages exhibited by batteries of the second cell type whose series resistance is within the first predetermined range of series resistance values; identifying the battery as having the first cell type if the series resistance is within the first predetermined range of series resistance values and the voltage is within the second range of voltages; and identifying the battery as having the second cell type if the series resistance is within the first predetermined range of series resistance values and the voltage is within the third range of voltages. The method Claim 1, further comprising: determining whether the series resistance of the battery is within a fourth predetermined range (40) of series resistance values exhibited by batteries of a first cell type; and identifying the battery as having the first cell type if the series resistance of the battery is within the fourth range. The method of Claim 2, further comprising: establishing an upper series resistance threshold (36) greater than the upper limit of the predetermined range; determining whether the series resistance of the battery (82) is greater than the upper series resistance threshold; and identifying the battery as having the first cell type if the series resistance value of the battery is greater than the upper series resistance threshold. The method of any preceding claim, further comprising: determining whether the series resistance of the battery (82) is within a further predetermined range (48) of series resistance values exhibited by batteries of a second cell type; and identifying the battery as having the second cell type if the series resistance of the battery is within the further range. The method of Claim 4, further comprising: establishing a lower series resistance threshold (44) less than the lower limit of the first predetermined range; determining whether the series resistance of the battery (82) is less than the lower series resistance threshold; and identifying the battery as having the second cell type if the series resistance value of the battery is less than the lower series resistance threshold. The method of any preceding claim, wherein the voltage-determining step comprises determining whether the voltage of the battery is less than, or greater than, a voltage threshold. The method of any preceding claim, wherein the step of measuring the series resistance of the battery comprises: applying a current differential of a known magnitude to the battery with a constant current source; measuring a voltage differential of the battery resulting from the applied current differential and thereby related to the series resistance of the battery. Apparatus for differentiating battery cell types between first and second candidate cell types which exhibit a common range (52) of series resistance values, said apparatus comprising: battery series resistance testing means (86) arranged for connection to a battery (82) for determining whether the series resistance of the battery lies within said common range (52) of series resistance values; battery voltage testing means (86) arranged for connection to the battery for determining whether the voltage of the battery lies within a first predetermined range of voltages (74) exhibited by batteries of the first cell type or within a second predetermined range of voltages (76) exhibited by batteries of the second cell type; and determining means (88) responsive to the battery series testing means (86) and the battery voltage testing means (86) for identifying the battery (82) as having the first cell type if the series resistance is within said common range (52) of series resistance values and the voltage is within said first predetermined range of voltages (74) and identifying the battery (82) as having the second cell type if the series resistance is within said common range (52) of series resistance values and the voltage is within said second predetermined range of voltages (76). The apparatus of Claim 8, wherein the determining means (88) is arranged for determining whether the series resistance value and the voltage value lie within any of a plurality of ranges (40, 48, 74, 76) of series resistance and voltage values, each range of series resistance and voltage values being associated with a cell type, and for identifying the battery as being of a cell type associated with the range in which the series resistance and voltage values lie. The apparatus of Claim 9 wherein the determining means comprises: series resistance comparison means (88) for comparing the series resistance value to an upper threshold (36) and to a lower threshold (44); and voltage comparison means (88) for comparing the voltage value to a voltage threshold (72); the series resistance comparison means and the voltage comparison means providing a first indication (92) corresponding to a first cell type when the series resistance value is greater than the upper threshold, providing a second indication (92) corresponding to a second cell type when the series resistance value is less than the lower threshold, and, when the series resistance value is between the upper and lower thresholds, providing the first indication when the voltage value is greater than the voltage threshold and the second indication when the voltage value is less than the voltage threshold. The apparatus of Claim 8, wherein the determining means further comprises table look-up means for associating a cell type with a plurality of possible series resistance and voltage values.

Inventors: STEPHENS CHARLES STANTON, STEPHENS, CHARLES STANTON
Applicants: HEWLETT PACKARD CO, HEWLETT-PACKARD COMPANY
Application year: 1994
Main classification: G01R31
IPC codes: G01R31, H02J7